1. Field of the Invention
The present invention relates to a slip frequency control for variable speed induction motors and, more particularly, to a closed-loop drive system for variable speed induction motors which maintains a constant slip frequency during both acceleration and braking, regardless of motor speed.
2. Description of the Prior Art
In an induction motor, the input energy is connected to the stator or the rotor circuit, typically to the field or stator winding. This eliminates the necessity for brushes which is a significant disadvantage in DC motor. In any event, the input energy causes a current to flow in the stator winding which creates a rotating magnetic field. The frequency of the input energy defines the synchronous speed of the motor.
An induction motor is capable of producing driving torque at any speed below the synchronous speed. If the rotor speed is greater than the stator frequency, braking torque is produced. In either event, the slip frequency is defined as the difference between the stator frequency and the actual rotational speed of the motor.
For a variety of reasons, it is desirable to maintain a constant slip frequency, regardless of motor speed. If, under given operating conditions, the slip frequency increases, the result is an increased motor current at the expense of higher output power. This increased current could become excessive and cause an overload shutdown of the power source.
Various schemes have been proposed to control the slip frequency in order to limit motor torque and current. Open loop frequency control methods are unsatisfactory where variable inertia loads are encountered or if unexpected friction loading occurs. Thus, phase-locked loop systems are preferred and such are known for induction motor control. Such systems generally employ a voltage controlled oscillator driven by a tachometer signal derived from the rotation of the motor shaft and the output of a slip frequency oscillator. Systems of this type are disclosed in U.S. Pat. Nos. 3,731,169 and 4,042,862. However, prior closed-loop drive systems have either had limited capabilities or have been overly complex and therefore impractical.